1. Field of the Invention
The present invention relates to a Light Emitting Diode (LED), and more particularly to a phosphor-free quantum dot/quantum well LED.
2. Related Art
Light Emitting Diode (LED) is a semiconductor light emitting component. Different from the traditional incandescent bulb which illuminates by a high current applied to heat the filament, the LED only requires a low current to emit the equivalent light. LED is based on the fact that in semiconductor materials when electrons are binding with holes the energy released is revealed in the form of emitting light. Due to having the advantages of small volume, long lifespan, low driving voltage, low power consumption, quick response, excellent shock resistance and good monochromaticity and on the like, the LED is widely used in light emitting component, such as various electrical appliances, information board communication products. Depending on the control of the chip type and process, various homochromous lights can be obtained.
Due to its power-saving feature, the LED can be used as lighting equipment instead of bulbs in the future. But now the white LED can not be widely used because of its luminance and price; however, white LED will contribute to the booming of the LED industry. At present, most of the white LED products emit yellow light by stimulating the phosphor with a blue LED. These two lights, after being mixed, can give a visual impression of white light. As the luminance of the blue LED is improved gradually, the application of the white LED has a bright future in the lighting market.
The development of high luminance LED vitalizes the LED industry. Especially, the success of the development of blue, green LEDs improves the luminous efficiency gradually. At present, the luminance of the LED can be higher than that of several candles and is still improved progressively. Due to the progressing luminance of the blue light, the application of the white LEDs produced by coating phosphor will be widely used in the lighting market. However, it is very difficult to control the light emitted by a white LED to be pure white, as it is produced by mixing the blue and the yellow light; instead, the light is approximately white with cyan or yellow halation, i.e. having unequal color temperature.
Presently, the commercially available mature product is the inorganic white LED developed by Japanese Nishia Chemical Institute. Referring to FIG. 1, a schematic structure view is shown. A blue die 10 with a wavelength of 460 nanometer is coated with a layer of yttrium aluminum garnet phosphor 20 which is excited by the blue LED to produce a yellow light with a wavelength of 555 nanometer complementary with the blue light. Then, the yellow and blue light complementary with each other is mixed according to the lens principle to obtain the desired white light. In this way, the manufacture cost of a white LED is low and the structure of the power loop is simple.
This white LED mainly comprises blue die and phosphor. However, because the lifespan of a phosphor is short, the lifespan of a LED is limited. Furthermore, as the light emitted by the LED does not contain red light, red light cannot be emitted when irradiating red objects, thus causing poor color rendering index.
To improve poor color rendering index, a method of mixing the lights emitted by of red, blue, green LEDs respectively is proposed to produce a white emitting light source. However, since LEDs of different wavelengths are not in the same area, but separated from one another with a fixed distance, the problem of poor color rendering index still exists.
At present, the advanced technology employs two epitaxy materials which produce lights of different wavelengths to produce a white light source by wafer bonding or wafer fusion. However, because the process of wafer bonding is employed in producing the light source, which results in an absorption to the double-chip interface, the emitting efficiency will be reduced, and the technological requirements and cost are indirectly raised, and also the difficulty in the manufacture is increased and the product yield is reduced.
Therefore, it is the problem to be solved that how to develop a phosphor-free LED having the features of high color rendering index, long lifespan, low cost and so on.